Optical fiber is finding increasing application in long-haul signal transmission, and many cable designs have been proposed or are available commercially. See, for instance, U.S. Pat. Nos. 4,078,853 and 4,241,979, coassigned with this.
Recently it has become apparent that optical fiber not only can advantageously be used as a long-haul (e.g., inter-city) transmission medium but potentially can also be used economically in the local distribution network (exemplarily including distribution, service or drop, and indoor cable), thus opening the possibility of end-to-end optical communications. However, the optical fiber cables that were designed for long-haul applications are typically not well suited for use in the local distribution network. For instance, such cables typically are designed to have medium to high fiber count (&gt;10 fibers), are frequently relatively rigid and have a relatively large bending radius, and tend to be relatively costly to manufacture. On the other hand, cable for use in the local distribution plant typically requires only low fiber count (e.g., .ltoreq.10 fibers), should be flexible, have a small bending radius, be usable in a variety of environments, be deployable by existing means e.g., by means of vibratory plough or by trenching, and advantageously be easy to manufacture.
The prior art does know several optical fiber cables that are designed for use in the local distribution network. For instance, S. Kukita et al, Review of the Electrical Communications Laboratories, Volume 32(4), pp. 636-645 (1984) review the design and performance of optical drop and indoor cables, and disclose, inter alia, an indoor cable comprising a coated fiber surrounded by a PVC sheath, with four steel wires embedded in the sheath. Such a design can be expected to have some shortcomings, including a need for electrically grounding the steel wires, and difficulty in achieving good coupling between the sheath and the steel wires, and may cause bending-induced stresses on the optical fiber.
An exemplary commercially available optical fiber cable that can find use in local distribution plant comprises a central (steel or polymer) strength member surrounded by a polyurethane jacket, a multiplicity of fiber-containing loose tubes stranded around the polyurethane jacket, polyester tape and a polyethylene inner jacket surrounding the tubes, and steel or Al armor and a polyethylene outer jacket surrounding the inner jacket. In such a structure the fibers cannot be on the neutral axis of the cable when the cable is bent, requiring stranding of the loose tubes, which in turn complicates manufacture. Furthermore, in such a design the strength member is not well coupled to the outside jacket.
Another commercially available optical fiber cable for use in the local distribution network has a core consisting of a central coated steel wire strength member and a multiplicity of polymer tubes arranged around the central strength member, each tube containing one buffered optical fiber as well as a moisture resistant filling compound. This core is surrounded by aramid yarn which is said to take up the greater part of any tensile load applied to the fiber. The yarn layer in turn is surrounded by a polyethylene jacket. Thus constituted cables are said to be useful as aerial or duct cable. In cable for direct burial, a polyurethane inner jacket takes the place of above referred to polyethylene jacket, and steel tape armoring and a polyethylene outer jacket surround the inner jacket.
In this cable structure too, the fibers cannot be on the neutral axis of the cable when the cable is bent, thus also requiring stranding of the loose tubes. Furthermore, axial stresses are poorly coupled from the outer surface of the cable to the central strength member.
Still another family of commercially available optical fiber cables for use in the local distribution network contains cables of the "loose tube" type in which one or more optical fibers are loosely surrounded by polyethylene or PVC jacket, the voids within the thus formed loose tube being filled with polymeric gel. Two aramid fiber or steel strength members are embedded in the jacket, thereby defining the neutral plane for bending. The cables are available with flame retardant jackets.
Among the drawbacks of such a design is the fact that bending of the cable will frequently cause it to undergo a substantial amount of twist. Furthermore, it is expected that in this design fibers and filling compound are exposed to relatively high temperatures during cable manufacture. In such a design, it may occur that vinyl-buffered fibers (a currently frequently used fiber type) may fuse to the PVC jackets.
Co-assigned U.S. patent application Ser. No. 770,041, filed Aug. 28, 1985, (now abandoned) discloses, inter alia, an optical fiber drop cable in which a plurality of optical fibers is loosely surrounded by impregnated glass tape, and a polymer jacket of generally rectangular cross section is disposed such that the fiber-containing core is located at the center of the cross section. Impregnated filaments (e.g., glass or aramid filaments) are gathered to form two strength members which are embedded in the polymer jacket and coupled thereto. This design too has one predetermined neutral plane in bending, and thus can be bent in only one plane.
In view of the desirability of the ability to provide an optical fiber transmission path onto customer premises, an optical fiber cable that can be easily adapted for indoor (including flame retardant, riser and plenum cable) as well as for outdoor use (drop cable or distribution cable, unarmored or armored, including rodent-resistantly armored), that is of small diameter, highly flexible, crush resistant as well as resistant to tensile loading, that can be terminated easily and conveniently, and that is easy to manufacture, would be of considerable interest. This application discloses such an optical fiber cable.